1. Field of the Invention
The present invention relates to a leadframe used for semiconductor devices and more particularly, to a leadframe used for plastic-molded packages of semiconductor devices such as LSIs.
2. Description of the Prior Art
A conventional leadframe 38 of this type has a body, a pair of rails extending along a longitudinal axis of the body and formed at each side thereof, and a plurality of patterned die-mounting structures as shown in FIG. 1. The structures are typically formed through an etching or stamping process. The leadframe 38 is usually made of an iron system alloy such as Alloy 42 or a copper system alloy.
Though the die-mounting structures are aligned along the longitudinal axis of the body at regular intervals, only one of the structures is shown in FIG. 1 for the sake of simplification of description.
As shown in FIG. 1, each of the die-mounting structures includes a square die pad 35 for mounting a semiconductor die or chip thereon, four die pad supports 37 for supporting the die pad 35, four sets of fingers for forming inner leads 31 and outer leads 32, and four sets of tiebars or dam-bars 34 for preventing leakage of molding material or compound during a molding process step.
The square die pad 35 is positioned at the middle point of the lead frame 38. Each of the sets of the fingers and corresponding one of the sets of the tiebars 34 are arranged to be opposite to each side of the die pad 35. Each of the tiebars 34 is formed to interconnect the adjacent two fingers the end-positioned finger and the body of the leadframe 38. Each of the sets of the tiebars 34 are aligned along each side of an imaginary square.
Each of the die pad supports 37 extends from a corresponding corner of the imaginary square to an opposing corner of the die pad 35 so that it mechanically connects the die pad 35 to the body. As a result, the die pad 35 is supported only by the die pad supports 37.
The four sets of the fingers are formed to protrude from the peripheral part toward the four sides of the die pad 35, respectively. The inner parts of the fingers, which are near the die pad 35, act as the respective inner leads 31 and the outer parts of the fingers, which are far from the die pad 35, act as the respective outer leads 32.
Three first pilot holes 36a and three second pilot holes 36b are formed in each of the rails to accurately locate the lead frame 38 on a lower mold of a molding die during a molding process step.
With the conventional leadframe 38 described above, chip mounting, wire bonding and plastic molding processes are carried out through the following process steps.
First, the leadframe 38 is placed horizontally on the lower mold (not shown) of the molding die at a given position as shown in FIG. 2A, and then, a semiconductor device or LSI chip 39 is mounted or fixed using silver paste 40, as shown in FIG. 2B.
Next, as shown in FIG. 2C, the inner leads 31 are electrically connected with bonding pads (not shown) of the LSI chip 39 through pieces 41 of a bonding wire, respectively.
Then, as shown in FIG. 2D, the chip 39, the die pad 35, the pieces 41 of the bonding wire and the inner leads 31 are transfer-molded by a thermoset molding material such as an epoxy resin, resulting in a plastic package 42. The tiebars 34 and the outer leads 32 are in the outside of the package 42.
After removing the tiebars 34 by cutting, a given metal film 43 is formed to cover the outer leads 32 by a plating process, and then, the outer leads 32 thus plated are partially cut in a lead trimming process, as shown in FIG. 2F.
Finally, the outer leads 32 thus trimmed are bent to given shapes in a lead forming process, resulting in a plastic-molded semiconductor device or LSI as shown in FIG. 2G.
A detailed description about the transfer-molding process shown in FIG. 2D is shown below referring to FIG. 3.
A molding material or synthetic resin 48 is preheated to a given temperature to melt and is temporarily stored in a pot 47 of a lower mold 46 of a molding die 45. The lower mold 46 has a cavity 52a. On the other hand, as shown in FIG. 2C, the leadframe 38 having the semiconductor chip 39 and the pieces 41 of the bonding wire is positioned on the lower mold 46 using the first and second pilot holes 36a and 36b.
Subsequently, an upper mold 49 of the molding die 45, which has a cavity 52b, is lowered to be coupled with the lower mold 46, providing a molding space made of the cavities 52a and 52b in the molding die 45. At this time, the body of the leadframe 38 is put between the upper and lower molds 45 and 46 so that the leadframe 38 is securely held. The LSI chip 39, the bonding-wire pieces 41 and the inner leads 31 are placed in the molding space.
A plunger (not shown) provided under the melted molding material 48 ascends to push out the material 48 stored in the pot 47 into a runner 50 of the lower mold 46. The melted material 48 flows in the runner 50 toward the lower cavity 52a as shown by an arrow A', passes through a gate 51 formed at an end of the runner 50 as shown by an arrow B', and go into the lower cavity 52a.
Since the runner 50 and the gate 51 are formed in the lower mold 46 and the leadframe 38 is placed at an interface of the upper and lower cavities 52a and 52b, the melted material 48 flows, first, into the lower cavity 52a as shown by an arrow C'. The melted material 48 in the lower cavity 52a then flows into the upper cavity 52b through gaps or openings between the die pad 35 and the peripheral part or body of the leadframe 38 as shown by an arrow D'. In other words, the melted material 48 branches to flow in the upper and lower cavities 52a and 52b.
The melted material 48 flowing in the lower and upper cavities 52a and 52b goes toward opposite ends of the cavities 52a and 52b, as shown by arrows E', and F' respectively, so that the cavities 52a and 52b are filled with the melted material 48.
During this filling process, the atmospheric air confined in the cavities 52a and 52b is discharged through an air vent 54 formed at the end of the lower mold 46 opposite to the gate 51.
The melted material 48 thus filled is then cured, and the semiconductor chip 39, the bonding wire pieces 41 and the die pad 35 are molded by the material 48, resulting in the plastic package 42 made of the material 48.
Subsequently, the upper mold 49 is raised to be apart from the lower mold 46, and the leadframe 38 with the semiconductor chip 39 thus molded are taken out from the lower cavity 52a.
The cured molding material 48 remaining at the runner 50 and the gate 51, that is, burrs or flushes, are then removed. Thus,the molding process step is finished
With the conventional plastic-molded semiconductor device described above, there are problems that visible voids or bubbles easily arise in the plastic package 48 after the curing process and that a failure called "no filling" of the cavities 52a and 52b occasionally arises due to insufficient injection of the molding material 48 into the cavities 52a and 52b.
The above problems are due to the following facts:
The gaps between the die pad 35 and the peripheral part or body of the leadframe 38 have become narrower due to increase in pin count and decrease in outer lead-pitch through down-sizing, so that the melted molding material 48 flows with difficulty from the lower cavity 52a to the upper cavity 52b through the gaps. Accordingly, there arises flow rate difference between the molding material 48 flowing in the lower and upper cavities 52a and 52b, resulting in the above failures.
For example, in the case of a Quad Flat Package (QFP) of an LSI with 304 pins or outer leads, the following testing results have been obtained:
When one hundred (100) semiconductor devices or LSIs are transfer-molded under the condition that each of gaps between the adjacent inner leads 31 and each of the gaps between the respective die pad supports 37 and the inner leads 31 adjacent thereto are both 100 .mu.m, the failure of the "no filling" is found in forty (40) of the devices thus molded. This means a very high failure rate of 40%.
To solve the above problems, an improved molding technique was developed, which is disclosed in the Japanese Un-Examined Patent Publication No. 2-186647.
With this molding technique, an upper mold of a molding die has a second gate at a position opposite to that of a first gate of a lower mold of the molding die, and a leadframe has a communication hole for communicating the first and second gates with each other when the leadframe is securely held between the upper and lower molds coupled together.
A melted molding material or synthetic resin flows through the first gate into a cavity of the lower mold and at the same time, the molding material flows through the communication hole and the second gate into a cavity of the upper mold. Therefore, the flow rates of the material in the lower and upper cavities becomes substantially equal to each other, avoiding the above failures.
However, there is a problem that since the molding material remaining at the second gate of the upper mold is difficult to be removed after a curing process, an additional process step is required for removing this remaining material on the upper mold, resulting in increase in fabrication cost.